The present invention relates to the field of cold cathode fluorescent lamp based lighting and more particularly to an arrangement in which low burst dimming duty cycles are supported.
Fluorescent lamps are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors. One particular type of fluorescent lamps is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ionize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to herein as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on. Preferably the CCFL is alternately powered from each end so as to maintain an even brightness across the lamp.
In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCD or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness, with the resultant AC signal preferably arranged to perform the aforementioned alternate powering. Even brightness across the panel and clean operation of the inverter system with low switching stresses, low EMI, and low switching losses is desirable.
In CCFL backlight applications the inverter system typically comprises a DC to AC controller in cooperation with external components operative to generate an AC power of a few hundred Volts to over one thousand Volts RMS so as to drive the lamps at a frequency of about 30 to 70 KHz. The DC to AC controller alone is sometimes referred to as an inverter. This high voltage raises certain safety issues, and as a result CCFL controllers typically provide an open lamp detection functionality. The open lamp detection functionality monitors a current, and optionally a voltage, associated with each one of the load CCFLs to detect if the CCFLs present an open circuit to the controller, referred hereinto below as an open lamp condition. Such an open lamp condition may be the result of missing, defective or burnt out lamps. Detection of an open lamp condition is normally accomplished by comparing the sensed lamp current to a pre-set threshold. If the sensed lamp current is lower than the pre-set threshold an open lamp condition is considered detected. Upon detection of an open lamp condition, the open lamp detection functionality is operative to shut down the controller so as to prevent the appearance of a high voltage AC signal without a valid load. In order to avoid transient response, typically the open lamp detection functionality requires an open lamp condition to be found for a plurality of cycles of the high frequency AC and the open lamp condition is cleared upon detection of a sensed lamp current indicative of normal operation. The plurality of cycles may be counted digitally in a counter, or by charging a capacitor with a known current source whenever an open lamp condition is detected. The lamp current indication of normal operation is in one embodiment a current greater than the pre-set threshold, and in another embodiment a separate higher threshold is provided to clear the open lamp condition.
In many such applications, such as backlighting for LCD based televisions, dimming is required to adjust the brightness of the backlight in order to produce satisfactory pictures in various ambient lighting conditions and various visual conditions. Dimming is typically achieved by one or both of analog dimming and burst dimming. Technically speaking, analog dimming controls the amplitude of the CCFL current, whereas burst dimming turns the CCFL on and off at a duty cycle so as to adjust the average brightness of the backlight over time. At any particular amplitude of the CCFL current, a lower duty cycle of burst dimming results in a dimmed light as compared to a larger duty cycle of burst dimming. The frequency of the burst dimming cycle is typically in the range of 150-250 Hz, and thus functions as an envelope for the higher frequency AC lamp voltage.
One limitation of CCFL is that the light may extinguish completely when the lamp is operated at a low current level. Furthermore, the efficiency of a CCFL at low current levels is lower than the efficiency of the CCFL at higher current levels. As a result a minimum lamp current limit is defined, which effectively limits the range of analog dimming. As a result, burst dimming is almost universally used, with the lamp current set to an optimum value and the brightness controlled by the burst duty cycle.
FIG. 1 illustrates a high level schematic diagram of a backlighting arrangement according to the prior art, illustrating an embodiment of the above open lamp condition detection. The backlighting arrangement comprises a plurality of CCFLs 10, a plurality of sense resistors RS each associated with a particular cold cathode fluorescent lamp 10, a resistor RD1 and a resistor RD2, a diode OR circuit 15 comprising a plurality of diodes DS each associated with a particular CCFL 10, a controller 20 and a step-up transformer 90. Controller 20 comprises a pulse generator 30, an open lamp detection functionality 35 and a lamp error amplifier 80. Open lamp detection functionality 35 comprises a comparator 40, a comparator 50, an OR gate 60 and an error cycle count functionality 70.
A received burst dimming pulse, denoted VBST is connected to an input of pulse generator 30. The output of pulse generator 30 is connected to a first end of the primary winding of step-up transformer 90, and the second end of the primary winding is connected to a first common point, illustrated as a chassis ground. A first end of the secondary winding of step-up transformer 90 is connected to a second common point, which may be different from the common point of the primary winding, and is illustrated as a local ground. A second end of the secondary winding of step-up transformer 90 is connected to a first end of resistor RD1 and to a first end of each of the plurality of CCFLs 10. The second end of each CCFL 10 is connected to a first end of the associated sense resistor RS and to the anode of a particular one of the diodes DS of diode OR circuit 15. The second end of each of the associated sense resistors RS are connected to the second common point.
The second end of resistor RD1 is connected to a first end of resistor RD2 and to the non-inverting input of comparator 40, denoted input VSNS of controller 20. The second end of resistor RD2 is connected to the second common point. A maximum voltage level, denoted VTH2, is connected to the inverting input of comparator 40. The output of diode OR circuit 15 is connected to the inverting input of comparator 50 and to the non-inverting input of error amplifier 80, denoted input ISNS of controller 20. An open lamp detection threshold current level, denoted VTH1, is connected to the non-inverting input of comparator 50.
The output of comparator 40 is connected to a first input of OR gate 60. The output of comparator 50 is connected to a second input of OR gate 60 and via an inverter to the clear input of error cycle count functionality 70. The output of OR gate 60 is connected to the input of error cycle count functionality 70. The output of error cycle count functionality 70, denoted FAULT, is connected to an input of pulse generator 30. A lamp current reference level, denoted IREF, is connected to the inverting input of error amplifier 80, and the output of error amplifier 80 is connected to an input of pulse generator 30. Error cycle count functionality 70 is illustrated as a digital counter, however this is not meant to be limiting in any way. In another embodiment error cycle count functionality 70 is implemented in an analog fashion with a capacitor arranged to receive a fixed current, and a fault signal will be output when a certain voltage level is reached.
VBST is illustrated as a received gating signal, however this is not meant to be limiting in any way. In one embodiment, VBST is derived from a received analog signal whose level is translated internally into the duty cycle for the burst dimming signal.
In operation, pulse generator 30 is operative to generate a pulse width modulated high frequency square wave gated by a low frequency burst dimming pulse VBST and thereby drive the primary side of step-up transformer 90. In one embodiment, pulse generator 30 drives an H-bridge switching arrangement connected to the primary side of step up transformer 90, as described in U.S. Pat. No. 5,930,121 to Henry, the entire contents of which is incorporated herein by reference. Step-up transformer 90 steps up the voltage of the signal received at the primary, and in cooperation with self inductance of step-up transformer 90 and parasitic capacitance of CCFLs 10, filters the resultant AC voltage to supply the AC voltage necessary for operation of CCFLs 10.
The voltage across the CCFLs 10 is divided by the voltage divider of resistors RD1 and RD2 and the divided voltage is presented via input VSNS to be compared with maximum voltage level VTH2. The current through CCFLs 10 are each sampled across the respective sense resistor RS, and the greater current is passed through diode OR circuit 15 and presented via input ISNS to be compared with open lamp detection threshold current level VTH1. The voltage representation of the current presented via input ISNS is further compared to lamp current reference level IREF, and any error is amplified and transmitted to pulse generator 30 which acts to increase or decrease the duty rate of the pulse width modulated output of pulse generator 30 so as to ensure that ISNS coincides with IREF when CCFLs 10 are being driven.
Open lamp detection functionality 35 is operative to compare the divided representation of the voltage across CCFLs 10 with maximum voltage level VTH2, and the representation of the greater current through the CCFLs 10 with open lamp detection threshold current level VTH1, and output an error signal to error cycle count functionality 70 whenever the divided representation of the voltage across CCFLs 10 exceeds maximum voltage level VTH2 or the representation of the greater current through CCFLs 10 is less than open lamp detection threshold current level VTH1. It is to be understood that the maximum voltage is developed across CCFLs 10 when no current is flowing, and thus voltage exceeding maximum voltage level VTH2 is indicative of an open lamp condition. Error cycle count functionality 70 is operative to count a predetermined number of error conditions, i.e. an error condition maintained for a predetermined number of cycles of pulse generator 30, and in the event that the error is maintained to assert the FAULT signal to pulse generator 30 thereby disabling pulse generator 30. In the event that the representation of the greater current through CCFLs 10 is greater than or equal to the open lamp detection threshold current level VTH1 before the FAULT signal is asserted, the output of comparator 50 clears error cycle count functionality 70.
One problem with the above described burst dimming is audible noise. When the CCFL is turned on and off electro-mechanical vibration occurs due to the sharp change of electro-magnetic force in the associated components, especially in the transformers. As indicated above, the dimming frequency is in the range of 150-250 Hz, which is well within the audible frequency range.
An effective method to reduce such electro-mechanical vibration is to control the profile of the burst lamp current so as to ramp up gradually when the burst dimming control changes from one state to another. Thus, when turning the lamp on, the current resultant from the high frequency AC voltage is ramped up to the nominal value, and when turning the lamp off, the current resultant from the high frequency AC voltage is ramped down from the nominal value until the lamp is off.
Controlling the profile of the burst lamp current successfully suppresses the audible noise, however because of the ramp up and the ramp down of the burst current, there will be a small period at the bottom of the ramp slope wherein the lamp current is lower than the pre-set threshold, described above in relation to VTH1, and open lamp detection functionality 35 will assert the FAULT signal thereby shutting down pulse generator 30. At low duty cycles of burst dimming the whole or a significant portion of the burst on period can result in a false open lamp protection of the inverter, as will be described below in relation to FIG. 2. Therefore most CCFL inverters cannot work at low burst duty cycles, specifically as low as several percent. This is disadvantageous as in today's market full darkness of the screen becomes one of the important performance requirements for high quality displays. To fulfill such a requirement the burst dimming operation has to be able to work stably at very small duty cycles down to zero percent.
FIG. 2 illustrates a graph of certain signals of the backlighting arrangement of FIG. 1, in which the x-axis represents a common time axis and the y-axis represents voltages in arbitrary units. The burst dimming signal, denoted VBST, in addition to VTH1 and ISNS are illustrated. An additional analog representation of error cycle count functionality 70 is shown, denoted Error_Count, and a trigger value above which assertion of the FAULT signal is asserted, denoted Fault_Level.
At each burst dimming pulse of VBST, the current through CCFLs 10 is ramped up to its nominal value as shown by the ISNS representation. The burst dimming is illustrated as being at a very low duty cycle, for instance around 10%, and therefore ISNS does not reach the VTH1 level before being ramped back down. Thus, for each burst dimming pulse, an additional error count is accumulated, as illustrated by the climbing of signal Error_Count. Error_Count is not cleared because of the failure of ISNS to be equal to, or greater than, VTH1. After a few cycles, Error_Count exceeds Fault Level, and the FAULT signal will thus be asserted, resulting in shut down of controller 20.
What is desired, and not provided by the prior art, is a backlighting arrangement that can provide a full range of burst dimming, while maintaining support for open lamp detection.